3 Answers
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It's best to stretch immediately after exercise. When you work out, your muscles are contracting and shortening. Stretching will help lengthen your muscles again, and your muscles will be warmest immediately after your workout. If you wait, your muscles may become stiff. Waiting a couple of minutes (while you cool down for example) is fine, but if you wait a few hours you'll need to warm up again, and you probably still won't get as deep a stretch as you would have after a full workout.